The accidental killing of Anton von Webern, over 60 years ago, was a pivotal moment in modern culture. Abstruse, obscure and painfully incapable of emotional expression, Webern had stepped outside to light an after dinner cigar procured by his ex-SS son in law on 15 September 1945, when an American soldier fired three times into the dark in the occupied Austrian village of Mittersill. "I have been shot," said the composer, stumbling back indoors. "It is over."

News of the tragedy was slow to travel and newsprint hard to come by. Months lapsed before any eulogy of Webern appeared in the musical press and when his first cantata was posthumously premiered at an international festival in London the following summer it was received with, at best, respectful bafflement, for nothing in the music was benign or ingratiating.

Webern wrote in tight little aphorisms, applying Arnold Schoenberg's 12-note method of composition with fanatical rigour to such random variables as rhythm, intervals and dynamic levels of loud and soft. Inspiration was anathema to Webern. All had to be strictly counted and numerically correct. If pleasure entered the process, it was the solitary satisfaction of making a line read the same forwards, backwards and upside down. Inverted by nature, Webern wrote music that turned in upon itself, rejecting every human value except absolute order.

To composers coming of age in postwar Europe, he was the perfect hero and patron saint: a composer who liquidated the cultural past with a clinical solution, his death an act of martyrdom. Pierre Boulez, in France, revered him above Schoenberg. Karlheinz Stockhausen, in Germany, claimed the discovery of Webern as his "greatest musical experience". In the second half of the 20th century, Webern defined and dominated modernity in music as Picasso had done in painting and Joyce in English literature.

No composer has ever achieved so much influence with so little music. Every note he wrote, played end to end, lasts little more than five hours. None of it is easy on the ear. It took Webern four years of study with Schoenberg in Vienna before he scratched out his first opus in 1908, a 12-minute Passacaglia for Orchestra.

In a tediously discursive era, brevity was his distinguishing feature. His only symphony is 10 minutes long, his string quartet eight. Concise as they are, his music requires unblinking concentration from the listener through a trail of disjointed plinks and energy spikes. The reward is neither instant nor necessarily sensual - which explains why Webern is the least loved of all great composers and the one held most to blame for alienating audiences from contemporary music. His impact is unmistakable. When the Hollywood director William Friedkin went searching for the scariest music on earth for his 1973 movie The Exorcist, it was Webern who chilled his marrow, choosing Five Pieces for Orchestra to stalk the realm between known and unknown (after a tranquillising burst of Mike Oldfield's Tubular Bells).